This invention relates to a microwave horn radiator assembly for radiating circularly polarized electromagnetic waves from a radiator at a front portion of the assembly, the assembly including an orthomode transducer providing a conversion between linearly and circularly polarized radiation. More particularly, the invention employs a septum increasing stepwise monotonically in height from a bottom wall to a top wall of a square waveguide to provide two rectangular waveguide ports at a back end of the assembly, opposite the horn radiator, to provide the conversion between linearly and circularly polarized radiation. The assembly also includes plural stepped waveguide sections forming an impedance matching section behind the horn radiator, and a set of capacitive posts between the matching section and the septum.
An antenna comprising an array of radiators providing circularly polarized radiation may be employed in numerous situations, including a mounting of the antenna on board a spacecraft to provide for communication between the spacecraft and a station on the earth. In the construction of the antenna, each of the radiators is formed as a part of a radiator assembly which includes microwave structures for converting a linearly polarized electromagnetic wave to a circularly polarized electromagnetic wave for transmission of a microwave signal, and for converting from circularly polarized radiation to linearly polarized radiation upon reception of a microwave signal. It has been the practice to employ an orthomode transducer to provide for the conversion between the linearly and circularly polarized radiation. The microwave structure for polarization conversion is substantially larger than the radiator itself. In a typical construction of orthomode transducer, perpendicularly oriented rectangular waveguides have been employed to provide for both right-hand and left-hand circularly polarized waves.
A problem arises in that the foregoing construction is inconvenient because of the excessively large size required of the microwave structure, including the orthomode transducer, which feeds electromagnetic power to the radiator, and which receives incoming signals from the radiator for each of the radiators of the array antenna. A further disadvantage in the foregoing construction is excessive complexity in the manufacturing process required to produce the microwave structure. Also, it is noted that a large bandwidth is advantageous in the use of communication equipment, and the foregoing construction has been disadvantageous in respect to a limitation of the maximum bandwidth available for communication. The physical size has been enlarged also because of a need for numerous tuning screws, the need for such tuning also complicating the manufacture and set-up procedure. Also, the radiator should be operated in such a fashion as to minimize mutual coupling between signals of the various radiators of the antenna array.